The invention relates to a sensor system for measuring pressure profiles. Such systems can be used wherever pressure distributions in one or two dimensions are to be measured. An important field of application is medicine. Here pressure profiles can be measured for example in urology, proctology cardiology and other disciplines, with the aid of catheters.
It is known that sensors are used to measure pressure. Such sensors are so small that they can be fitted for example into a catheter (cf. e.g.: R. L. Smith, S. D. Collins: Sensor Design and Packaging in W. Gxc3x6pel, J. Hesse, J. N. Zemel: Sensors, Volume 1, VCH Verlagsgesellschaft, Weinheim, 1989, pages 79-106).
What is disadvantageous about such sensors is that pressure can only be measured in one place with their help. In order to record pressure profiles, pressure sensor arrays would have to be used, or the sensor would have to be moved during the measurement process.
Sensor arrays are technically complicated and expensive; the movement of sensors is difficult to carry out in many applications, or even impossible.
Therefore, the object underlying the invention is to realise a measurement system in which a pressure profile can be measured, without the catheter having to be moved during this process. The use of expensive sensor arrays is intended here to be dispensed with.
This object is achieved according to the invention in that the measurement catheter is formed as a tubular flexible hollow body (1) of length Lxe2x80x94called xe2x80x9ctubexe2x80x9d belowxe2x80x94(FIG. 1b). The tube has an inner circular, oval, rectangular or other shaped cross-section of size A. An external pressure load p(x) (FIG. 1a) is represented on the tube (1) as a cross-sectional function A(x) (FIG. 1c). This comes about by compression of the tube (1), which is caused by the difference between external and internal pressure.
The correlation between the external pressure load p(x) and the cross-sectional function A(x) can be varied in the sense of a measurement range adjustment through alteration of the pre-set pressure po in the interior of the tube (1).
The local cross-section A(x) is scanned by the tube being filled continuously from one side (x=0) with a liquid substance (I) which displaces substance (II) up to filling length xA (FIG. 2).
For the correlation between the filling length xA and the filling volume Vf the following is true:
dxA=dVf/A(xA)
From this follows the filling volume Vf (substance I):       V    f    =            ∫              x        =        0                    x        =                  x          A                      ⁢                  A        ⁢                  (          x          )                    ⁢              ⅆ        x            
The filling length xA can be measured according to different methods:
measurement of the electrical resistance
measurement of the electrical capacity
measurement of the acoustic resonance.
The filling length xA can be ascertained in a simple manner through measurement of the electrical resistance with the aid of a measuring instrument (4).
In one of the possible measuring methods, the measuring instrument (4) is connected via the electrical supply lines (5) with the tube (1xe2x80x2) (FIG. 3). The measurement can be taken with direct current or alternating current at electrical voltages of U greater than 10 mV.
The tube (1xe2x80x2) of length L has an electrical resistance in the range between 102-107 xcexa9. This is achieved by the tube (1xe2x80x2) being manufactured from electrically conductive plastics material, or by the inner surface being coated with an electrically conductive film.
To fill the tube (1xe2x80x2), a substance (I) with a specific resistance xcfx811 is used. The displaced substance (II) has a specific resistance xcfx812. The following must be true:
xcfx812 greater than  greater than xcfx811
Substance (I) can be any kind of electrically conductive liquid (e.g. salt solution: NaCl, KCl . . . ) Substance (II) can be air, some other gas, but also a liquid (e.g. oil) which fulfills the condition xcfx812 greater than  greater than xcfx811.
If, per length unit along the tube, the resistance of substance (I) is very much lower than the resistance of the tube, in the region filled with substance (I), the electrical resistance of the tube is approximately short-circuited.
As the tube is filled, therefore, there arises a measurable electrical resistance R as a function of the filling volume. Since the filling length xA increases more quickly in the region of smaller cross-sections A than in regions of greater cross-sections, in the course of function R (Vf) larger amounts of increase occur in the regions with small cross-sections. The differential quotient dR/dVf is thus a representation of the cross-sectional function A (XA):             ⅆ      R              ⅆ              V        f              -      (          -              1                  A          ⁢                      (                          x              A                        )                                )  
For the measurable resistance R, the following statement arises:   R  =                              R          f                L            ·              x        A              +                            R          s                L            ⁢              (                  L          -                      x            A                          )            
In this, Rf is the electrical resistance of the filled tube, Rs is the resistance of the empty tube and L is the tube length.
For Rf less than  less than Rs, for the course of the electrical resistance, R arises as a function of the filling length xA      R          R      s        ≈                    -                  1          L                    ⁢              x        A              +    1  
and from this for xA      x    A    ≈      L    ⁢          xe2x80x83        ⁢          (              1        -                  R                      R            s                              )      
The course of the pressure profile p(x) is represented by the amount of the differential quotient dR/dVf over the filling length xA.
A similar measuring principle arises if the tube (1) has a very high electrical resistance Rso. Into the inner cavity of the tube (1) (FIG. 4a) is inserted an electrical conductor (28) (e.g. resistance wire) with an electrical resistance Rsd (FIGS. 4b+c). For the resistances Rso and Rsd the following should apply Rsd less than Rso.
The electrical conductor (28) can be inserted e.g. in the form of a loop (e.g. wire loop) (FIG. 4b).
When the tube (1) is filled with a substance (I), which, in comparison with the electrical conductor (28), has a low specific resistance, the resistance portion of the electrical conductor (28) between x=0 and x=xA is approximately short-circuited. Analogously to the above represented example, the following arises for the electrical resistance between the connection points (30) and (31) which is measurable with the resistance measuring instrument (4):             ⅆ      R              ⅆ              V        f              -      (          -              1                  A          ⁢                      (                          x              A                        )                                )  
The pressure profile can be measured in the manner already described.
It is also possible to insert just one electrical conductor (28xe2x80x2) (e.g. resistance wire) into the inner cavity of the tube (1). The circuit is closed with the aid of a low-impedance electrical conductor (29) (e.g. wire) which can be realised with or without external electrical insulation.
The pressure profile can also be measured on the basis of a capacity measurement. For this purpose, the tube (1xe2x80x3) is manufactured from an electrically insulating material (FIG. 5a). The capacity can be measured with the aid of a capacity measuring instrument (7) between the electrically conductive filling (I) and an electrically conductive medium (18), which surrounds the tube (1xe2x80x3).
The measurable capacity C increases as a function of the filling length xA.
FIG. 5b shows the possible way of providing contact for substance (I) with the aid of a short pipe (16) which is inserted into the end of tube (1xe2x80x3).
It is also possible to measure the capacity between the electrically conductive substance (I) and an electrically conductive coating (6), which is located on the surface of the tubular flexible hollow body (1xe2x80x3) (FIG. 6).
For the measurable capacity C the following arises:
C=cfxA/L
Here cf is the capacity of the filled tube.
xA arises as xA=L C/Cf 
The differential quotient dC/dVf is the representation of the pressure profile over the filling length xA.
The filling length xA can also be detected by an acoustic method. If air or some other gas is used as substance (II), through acoustic excitation of this gas column the unfilled length (Lxe2x88x92xA) of the tube (1) can be measured by determining the resonance frequency.
The advantages achieved with the invention lie in particular in the fact that pressure profiles can be measured without the measuring catheter having to be machine-operated. In addition to this it is possible, through periodic enlargement or reduction of the filling height xA to scan specific regions. Through variation of the counter pressure, information can be obtained about the solidity of the material exercising pressure.